1. Field of the Invention
The invention relates to improvements in switching power supplies for providing D.C. electrical energy to one or more electrical loads. More particularly, it relates to a switching power supply in which certain high frequency noise signals produced by the switching component of such supplies are effectively isolated from the load. Thus, the switching power supply of the invention is particularly useful in low signal applications.
2. Discussion of the Prior Art
In blood cell counters and a variety of other electronic instrumentation and control applications, it is necessary to supply well-regulated D.C. electrical energy to a plurality of electrical loads. Such loads may not only be physically located remotely from one another, but may also have voltage and current requirements which differ markedly from one load to another. To accommodate such loads, it is common to use a single bulk D.C. power supply to feed a common D.C. bus which, in turn, feeds a plurality of local D.C. to D.C. converter modules which are adopted to provide the voltage and current required by each respective load. Preferably, each D.C. to D.C. converter is physically located in close proximity to the load(s) supplied by it. This reduces voltage drops due to wiring resistance and permits precise regulation of load voltages despite input voltage fluctuations and/or changes in the magnitude of the current demanded by the load. Typically, a linear power supply feeds the D.C. bus supplying the D.C. to D.C. converters.
In a conventional linear power supply, A.C. line voltage is applied to the primary winding of a transformer. The respective outputs of one or more secondary windings of the transformer are then rectified and capacitively or otherwise regulated to reduce output ripple to the degree necessary to supply the voltage and current requirements of a particular D.C. load. While linear power supplies are capable of providing a well-regulated output with little if any electrical noise, they tend to be physically bulky, heavy and expensive to build.
Owing to the above-noted limitations of linear power supplies, systems engineers have increasingly turned to switching power supplies for a variety of applications. As used herein, the phrase "switching power supply" is to be construed broadly and refers to any power supply having an output which is regulated at least in part by altering the conductive state of a semiconductor switching device, such as a transistor, to control the width, duty cycle or frequency of a successive series of pulses. Because switching power supplies do not rely primarily on capacitors or other passive components for output regulation, they are typically more efficient, lighter in weight and less expensive than linear power supplies of comparable output power rating. However, the performance of switching power supplies entails certain characteristics which heretofore have rendered such power supplies unsuitable for some applications. Notably, the operation of the semiconductor switching devices in switching power supplies tends to generate a significant amount of high frequency electrical switching energy. This electrical switching energy creates a problem in extremely noise-sensitive applications, such as in many medical instruments in which signals of small magnitude are measured or otherwise processed. In such devices, high frequency electrical switching energy tends to mask the signal of interest and thereby adversely impacts the performance of the device. Normally, this problem can be overcome by A.C. coupling the positive and negative inputs of each D.C. to D.C. converter directly to one common ground. However, such an arrangement is not satisfactory when dealing with extremely noise-sensitive loads because of the tendency of noise to conduct along the common ground path and interfere with the proper operation of the loads.